Could Scanning a Document Cause My External Hard Drive to Disconnect?

The fact that Windows reports the device as healthy in Device Manager does not necessarily mean the disk is readable. Device Manager may only be detecting the USB enclosure, while the actual storage device is failing to initialize. Try a different cable and, if it is a desktop-style external drive, its original power adapter as well. Do not format or initialize it if Windows prompts you.

Scanning with the printer almost certainly did not disable the hard drive. The timing was probably a coincidence, and even scanning a document directly to that drive would not normally cause this. Since it appears in Device Manager but not Disk Management, the problem may be with the drive enclosure, USB-to-SATA bridge, cable, or the drive itself.

If the data is important, avoid repeatedly reconnecting or modifying the drive. You could try removing the drive from its enclosure and connecting it directly to a computer's internal SATA port, assuming you are comfortable doing that. If it still does not appear in Disk Management, professional data recovery may be the safest option.

This is also a good reminder not to keep the only copy of important files on one external drive. Once you regain access, use at least one additional backup, such as another drive or reliable cloud storage. A single drive can fail without warning, regardless of what was happening on the computer at the time.